Osmotic concentration cells are often used to concentrate food products, such as fruit juices, by selectively removing water. It has been recently found that many consumers are willing to pay a premium for high-quality fruit juices. This is evidenced by increased sales of single-strength products (i.e., products that have not been concentrated) in a market dominated by frozen concentrates. Therefore, there is a need in the art to be able to concentrate various food products by selectively removing only water, such that upon reconstitution, the product provides all the qualities of unconcentrated juice. This avoids the unnecessary expense of transporting excessive quantities of water.
In osmotic concentration cells, water diffuses from the food product to be concentrated through a semipermeable membrane into a solution having a much lower water content. Osmotic concentration systems are of interest to concentrate food products without use of heat that may damage the food product, such as a fruit juice. This prevents much of the quality degradation associated with evaporative processes. Food products of particular interest for osmotic concentration include other fruit and vegetable juices, agricultural products such as aloe vera, and pharmaceutical and biopharmaceutical products.
Most juice concentrates are made using evaporators. This is the source of quality problems with concentrates due to loss of volatile fragrances, denaturation and self-oxidation of the product. Volatiles are lost during the evaporative process because volatile agents have a lower boiling point than water. Some of the volatiles can be recovered by distillation of the vapor, however this system is, at best, only partially successful and it adds tremendous costs to the overall process. Loss of volatile degrades product quality because the volatile species are responsible for the fragrance associated with various products, such as fresh juice. Self oxidation is primarily a problem for juices. The term refers to enzymatic destruction of colors and flavors in the juice by naturally-occurring enzymes. The enzymes are derived from the juice and are largely inactive at refrigeration temperatures. However, such enzymes can become active at the higher temperatures associated with commercial evaporation.
Previous attempts to solve the problems of loss of volatiles and self-oxidation during concentration of juices include various cold concentration techniques. For food products, cold techniques include freeze concentration, reverse osmosis (RO) and direct osmotic concentration (DOC). Freeze concentration can eliminate self-oxidation problems and produce a higher quality juice than evaporators. However there are problems associated with freeze concentration, including: (1) only clarified juices can be concentrated, (2) some minor juice constituents are lost, and (3) capital costs of a commercial scale freeze concentration plant is many times that of an evaporation plant. Accordingly, the costs for freeze concentration are prohibitive, making this process not economically feasible.
RO techniques have lower capital and operating costs than freeze concentration. However, RO capital costs still far exceed evaporation techniques capital and operating costs. RO operates by squeezing water out of a juice through a semipermeable membrane (a thin polymer which has been manufactured to be porous). In semipermeable membranes, the pore structure is small enough that small molecules, such as water, can pass through, while larger organic molecules are prevented from passing. Drawbacks to RO are that the membranes foul rapidly and extremely high pressures (e.g., 3000 psi) are needed to form a juice concentrate.
Direct osmotic concentration (DOC) has been studied less than other cold concentration processes. DOC uses semipermeable membranes. However, instead of squeezing water out with pressure, DOC uses a solution with a lower mole fraction of water to pull water out of a product. This solution with a low mole fraction of water is an osmotic agent (OA). In DOC, any water pulled from the product into the OA must be subsequently removed from the OA, preferably by evaporation, in order for the OA to be recycled. Evaporation of the OA does not effect product quality because the product itself is not heated. Since DOC requires an evaporation step, its energy requirement is similar to evaporative concentration.
Concentrating products containing large organic molecules and solutions with suspended solids (e.g., pulp from orange juice) creates severe membrane fouling problems, particularly in RO systems. Even when a product is first filtered to remove suspended solids, there is severe fouling due to deposition of films on the surfaces of the semipermeable membrane. Such films are composed of large molecules such as polypeptides, pectins and polysaccharides. Large organic molecules adhere to the surface of the membrane due to Van der Waals attraction. After attachment, the large molecules slowly denature and unfold and then form multiple hydrogen bonding and multiple ionic interactions with the membrane. Therefore, it is desirable to induce a high degree of turbulence at the membrane surface to sweep these molecules away from the membrane surface before they adhere due to stronger bond formation.
Film breakup in RO systems has been attempted in, for example, Cross, "Membrane Concentration of Orange Juice" Proc. Fla. State Hort. Soc. 102, 1989. If the velocity is kept above 1 m/sec in tubular RO systems, enough turbulence is produced that may unfiltered juices can be concentrated up to 30% dissolved solids (30 Brix). Above 30 Brix, turbulence can no longer pull attached molecules away from membrane pores because pressures needed to push water out of the juice at Brix levels above 30 (i.e., greater than 5 MPa) produce pressure gradients at surfaces of membranes greater than 1000 GPa/m. Tubular RO is the RO configuration most resistant to fouling, but requires the greatest amount of space. Plate and frame configurations have greater surface fouling, but are more economical systems to build. Hollow fiber configurations are most susceptible to fouling.
Another method for concentrating juices to a high Brix level in an RO system first removes all large molecules by ultrafiltration before RO concentration. This method allows for concentration of a 60 Brix juice. However this method is relatively expensive, is difficult to maintain in view of the ultrafiltration step, and is energy intensive.
Flux of water in both RO and DOC systems is controlled by both pressure and osmotic concentration gradients. This can be quantified as: EQU F.sub.w =k.sub.w [(P.sup.I -q.sup.I)-(P.sup.II -q.sup.II)]
wherein F.sub.w is the water flux, k.sub.w is a constant for flux through a particular membrane, P is pressure, q is the osmotic potential and the I and II superscripts refer to solutions on each side of the membrane. Osmotic potential is a measure of the tendency of water to diffuse through a membrane from a region of high water concentration to a region of lower water concentration. In general, a good approximation of osmotic potential is q=cRT, wherein c is the molar concentration of non-water species in a solution, R is a gas constant, and T is the absolute temperature.
In RO systems, pressure differences are large but the osmotic potential of a permeate (the solution water is pushed into) is essentially zero. In DOC systems, pressure differences are negligible and flux depends on the difference in osmotic potential. In RO systems, the value of the flux constant k.sub.w is determined by molecularscale hydrodynamic resistance to flow through the membrane pores. To minimize this resistance, RO systems use extremely thin membranes (e.g., less than 0.01 mm thick) supported by a porous backing wherein the backing provides structural properties to the membrane to be able to withstand high applied pressure. The pores in the back are much larger than pores in the membrane, hence the backing contributes little to flow resistance across the membrane.
DOC k.sub.w values are primarily controlled by diffusion rates. Diffusion rates are less dependent upon pore diameter than hydrodynamic flow rates. Thus, in DOC cells, the presence of a membrane backing can severely reduce flux rates. Therefore, DOC membranes and their backings, if any, must be as thin as possible.
Further resistance to water flux in DOC systems occurs due to boundary layers on the surfaces of membranes. Boundary layers are thin regions in the fluid next to a membrane surface where mixing with bulk fluid is ineffective. An impact of boundary layers is creation of an extra region that water must diffuse through. Boundary layers are often avoided by using high fluid velocity (turbulent flow if possible) to sweep away boundary layers. Therefore, it is important in this art to design a flow cell configuration to avoid dead spots in fluid flow near membranes. The present invention was made, in part, to solve some of the problems of DOC systems for juice concentrators and dialysis systems.
In the wine industry, there is a need to provide a low alcohol wine to have an alcohol content below 10% to satisfy certain governmental regulations and to satisfy a consumer demand for a wine beverage with full bodied taste and body with less alcohol. There is also a need in the wine industry to improve thin wine fermentations that sometime result and produce a low grade wine. If a thin, low grade wine could be concentrated in both its water and alcohol contents, then it could be transformed into a premium wine and command a higher market price. There is also a need to improve the quality of various grape juices used for wine fermentation to use a lower grade juice to make a premium wine. The use of the inventive device according to the inventive process can satisfy these wine industry needs.
There are a number of processes for reducing the alcohol content of fermented drinks, such as wine, beer and champagne. One technique subjects the fermented drink to fractional distillation (usually under a vacuum) in order to remove some ethanol. Fractional distillation usually removes some of the taste and aroma substances from the drink and usually requires addition back to the drink of taste and aroma substances removed. In spite of extraordinary efforts involved in fractional vacuum distillation and rapid evaporators (e.g., thin film evaporators) this process produces poor quality products because the product often does not have the same or nearly the same taste as the original product. In another process, ethanol in a fermented drink may be extracted by means of organic solvents. However, use of organic solvents in this manner would violate many food processing regulations and the taste of the drink is often severely affected. It is known that the flavorings and aroma agents that form the bouquet and other such substances of a wine beverage include about 400 to 600 different substances, such as higher alcohols, esters, aldehydes, ketones, lactones, etc. The total volume of such substances is only about 0.5% to about 1% of the wine volume. However many of these substances are volatile with their boiling points being below and above the boiling point for ethanol. These substances also overlap the corresponding properties of ethanol with regard to their extraction behavior.
Most recent efforts for producing low alcohol beverages have focused upon a dialysis technique to lower alcohol content across a membrane. For example, in U.S. Pat. No. 4,804,554, a hydrophillic membrane is used and the osmotic agent (OA) or "dialysate" is composed of fractionally distilled beverage without alcohol or flavor substances. After each use, the dialysate is "stripped" of alcohol (and other flavoring agents) by fractional distillation. In U.S. Pat. No. 4,581,236 the dialysate is again the alcohol-free beverage. In U.S. Pat. No. 4,664,918, the dialysate is the alcohol-free beverage or an alcohol-free fruit beverage. Use of a dealcoholized product as the OA, however, tends to dilute the product being dialyzed. This happens because removal of alcohol from the juice to be used as the OA lowers the osmotic potential of the solution, with the result that when the OA is introduced into the membrane cell, water is pulled from it into the dealcoholized product.
Dilution of fermented beverages during alcohol removal is undesirable because, by itself, removal of alcohol decreases the sensory impact of beverage flavors. A dilution of the product further decreases product flavor.
In another technique, U.S. Pat. No. 4,867,997 describes a complicated high-pressure extraction process in the presence of carbon dioxide in a supercritical fluid state. Therefore, there is a need in the art to find an improved process to make a low alcohol fermented beverage (e.g., wine, beer or champagne) that retains the flavor and aroma characteristics of the original beverage. The present invention was made to achieve such a goal.